Signalling in Modern Telecommunications Systems
In modern switched telecommunication systems (in particular, modern PSTNs) it has become common practice to provide two related but separate network infrastructures: a bearer or transmission network for carrying end-user voice and data traffic, and a signalling network for controlling the setup and release of bearer channels through the bearer network in accordance with control signals transferred through the signalling network. In practice such signalling networks comprise high-speed computers interconnected by signalling links; computer programs control the computers to provide a set of operational and signalling functions in accordance with a standardized protocol. One example of such a signalling protocol is the afore-mentioned Signalling System No. 7 (SS7) which is being extensively deployed for control of telephone and other data transmission networks. An SS7 network basically comprises various types of signalling points, namely, signalling end points (SEPs) and signalling transfer points (STPs) interconnected by signalling links, the SEPs being associated for example with respective service switching points (SSPs) of the transmission network, and with service control points (SCPs).
Referring to FIG. 1, an SS7 network 10 is shown inter-communicating three signalling end points constituted by two service switching points SSPs 11 (between which extend speech circuits 12 of a transmission network not further illustrated) and a service control point SCP 13. The SCP serves to implement particular services (sometimes called IN, or Intelligent Network, services) in response to service requests received from an SSP, such a service request being generated by an SSP upon certain trigger conditions being met in the SSP in respect of a call that it is handling. A typical service may involve the translation of the dialled number (called party number) to a different number, the SCP returning this latter number to the SSP to enable the latter to complete call setup.
The SS7 network 10 includes two pairs 14 of signalling transfer points STPs, and a plurality of link sets 18 interconnecting the SSPs, SCP and STPs into a redundant network. Each signalling link set 18 is made up of one or more individual signalling links, the number of signalling links in a link set being chosen to provide appropriate capacity for the level of signalling traffic expected. The redundancy provided in respect of the STPs and links is to ensure that the failure of a single component of the network core does not cause the whole network to fail.
It should be noted that an SS7 network will typically comprise more STP pairs, SSPs and SCPs than illustrated. Service control functionality, as well as being provided in an SCP, can be provided in an Adjunct directly connected to an SSP.
Messages traversing the links of the network may be any of a large number of different types, depending on the nature of the call to which the message relates and the function specified by the message.
The SS7 Architecture
In order to facilitate an understanding of the present invention, a brief review will be given of the layered structure of the SS7 architecture and of the messages passed over the links of the network 10 to implement the SS7 architecture.
FIG. 2 illustrates the SS7 architecture. Levels 1 to 3 (referenced 21, 22, 23) form the message transfer part (MTP) 24. The MTP 24 is responsible for transferring signalling information between signalling points in messages. Level 4 (not referenced as a whole) comprises circuit-related user parts, namely ISDN User Part (ISUP) 26 and Telephone User Part (TUP) 27. These user parts define the meaning of the messages transferred by the MTP 24 and provide functionality to the users of SS7 (block 29). The user parts 26 and 27 are specific to particular types of circuit-related applications as indicated by their names. In fact, the ISUP is the most important user part, the TUP being a subset of ISUP and having been largely replaced by the latter. Most inter-exchange signalling, such as between SSPs 11 in FIG. 1, uses ISUP messages.
As well as the circuit-related user parts, SS7 level 4 also includes functional elements defining a general protocol for non-circuit-related information, such as operations, maintenance and administration information or network database information. The main functional element in this Level 4 protocol is the Transaction Capabilities (TC) 30 which sits on top of a Signalling-Connection-Control Part (SCCP) 31 and beneath a TC Users element 32.
The SCCP 31 actually forms part of the transfer mechanism for non-circuit-related applications, combining with MTP 24 to provide transfer mechanisms (both connectionless and connection oriented) meeting the Open Systems Interconnection (OSI) Layer 3/4 boundary requirements. TC 30 itself comprises two elements, namely an intermediate-services part (ISP) and a transaction-capabilities application part (TCAP); ISP is only used for connection-oriented services. Users of the SCCP/TC stack include the INAP (Intelligent Network Application Part) 32 and MAP (Mobile Application Part) 33. With reference to FIG. 1, messages passed between an SSP 11 (FIG. 1) and SCP 13 will be INAP messages using SCCP/TC (in fact, such messages are generally concerned with real time query/response transactions for which a connectionless service is most appropriate so that only the TCAP part of TC is used). Some inter-exchange signalling may also use SCCP/TC messages where, for example, the purpose of the signalling is service related rather than circuit related. ISUP may also use the SCCP for certain messages.
Considering the MTP 24 in a little more detail, Level 1 (reference 21) defines the physical, electrical and functional characteristics of the transmission path for signalling; typically, this will be a 64 kbit/s slot in a multiplexed stream. MTP Level 2 (reference 22) defines the functions and procedures for the transfer of signalling messages over a link between two directly-connected signalling points. MTP Level 3 (reference 23) provides functions for the reliable transfer of signalling information from one signalling end point to another. Thus, Level 3 is responsible for those functions that are appropriate to a number of signalling links, these being separable into signalling-message handling functions and signalling-network management functions.
When considering the passing of messages over a single link, it is the combination of Levels 1 and 2 that provides for the reliable transfer of signalling information. The Level 2 functions provide a framework in which the information is transferred and performs error-detection and error-correction processes; the Level 2 functions are carried out afresh on a link-by-link basis. At Level 2, information is seen as being transferred between signalling points in messages known as "signal units".
The general form of a signal unit 40 is shown in FIG. 3. As can be seen, a field 41 carrying message/data is encapsulated in a Level 2 framework comprising the following fields: a flag field; a backward sequence number field (BSN); a backward-indicator bit (BIB); a forward sequence number field (FSN); a forward-indicator bit (FIB); a length indicator field (LI); a spare field (SP); a check field; and a terminating flag field. The FSN, FIB, BSN, BIB and check fields provide error correction functionality at link level in a manner well understood by persons skilled in the art.
There are three types of signalling unit:
MSU--the Message Signal Unit--MSUs carry all service/application data sent on the SS7 network. The amount of data per MSU is limited to 273 octets maximum. PA1 LSSU--the Link Status Signal Unit--LSSUs carry information relating to the status of the link and are therefore concerned with Level 2 functions. Normally, LSSUs are only seen during the initial alignment procedure when a link is brought into service but are used at other times, for example, to stop the flow of signal units when processors are busy. PA1 FISU--the Fill-In Signal Unit--When no MSUs or LSSUs are to be sent, a signalling point continually sends FISUs. FISUs carry basic Level 2 information only, for example, the acknowledgement of the last MSU (field 41 is empty). PA1 an input and an output to which respective portions of the link can be connected, PA1 message path means extending between the input and output and comprising: PA1 selection means for selecting particular messages passing along the message path means according to at least one predetermined criterion, the selection means generating a modification signal in respect of each said particular message concerning a modification to be effected thereto; and PA1 modification means responsive to the modification signals for effecting the desired modifications to said particular messages in passage through the message path means; PA1 loopback means for connecting an output of the transmit means to an input of the receive means, PA1 insertion means for inserting predetermined messages in the message path means and causing them to circulate therearound, and PA1 comparison means for comparing the original form of the predetermined messages with the circulated messages after the latter have undergone at least one traverse of the message path means, the comparison means only permitting the un-bypassing of the message path means by the bypass means in the absence of unexpected differences between the compared messages. PA1 delay monitoring means for deriving a delay indication indicative of the delay experienced by messages passing through the message path means, and PA1 delay control means for reducing this delay upon the delay indication indicating that the delay has become too large. PA1 means for generating a time reference signal indicative of a current time for the apparatus, PA1 timestamp means for associating a timestamp with each message received by the receive means, this timestamp being derived from the time reference signal and indicating the current apparatus time at which the message is processed by said receive means, and PA1 means for generating the said delay indication as the time difference between the current apparatus time and the time value of the timestamp associated with the message at or adjacent the head of the queue means;
The length indicator (LI) within each message indicates the signal unit type as follows: LI=0 means FISU; LI=1 or 2 means LSSU; and LI=3 or more means MSU.
FIG. 3 further illustrates at 42 the basic format of an MSU; as can be seen, it comprises a service information octet SIO of 8 bits and a signalling information field SIF of 8n bits, where n is a positive integer. The SIO field includes a Service Indicator sub-field that defines the user part or equivalent appropriate to the message. The SIF contains the information being transferred and will generally include a routing label 43 comprising a 14-bit destination point code indicating the destination signalling end point, a 14-bit originating point code indicating the originating signalling end point, and a 4-bit signalling link selection field for specifying a particular link in cases where two signalling points are linked by a multiple-link link set. The MTP 24 is not aware of the contents of the SIF other than the routing label.
As an example of the information that may be borne by an MSU, FIG. 4 illustrates the general format of an ISUP message. As can be seen, in addition to the routing label 43, an ISUP message comprises a circuit-identification code (CIC) 44 indicating the number of the speech circuit between two exchanges to which the message refers, a message type code 45, and a number of parameters organised into three parts 30 46, 47, 48 according to type. Mandatory parameters of fixed length are placed in the mandatory fixed part 46. Mandatory parameters of variable length are placed in the variable mandatory part 47. Optional parameters are placed in the optional part 48. A typical ISUP message is the initial address message (IAM) which is the first ISUP message sent out when a call is being set up; the IAM will contain the required address (e.g. the digits dialled by the calling customer) and it results in a seizure of a circuit by each exchange along the route to the called-party exchange.
FIG. 5 illustrates the format of another message type that may be carried in the SIF, this time an SCCP message. The message format is, in fact, very similar to that of FIG. 4 but without the CIC field (as already indicated, SCCP messages generally concern non-circuit related messaging). A typical use for SCCP messages is to carry query/response messages between a SSP and an SCP, this being done in SCCP messages of the Unitdata type that utilise a connectionless service.
Liberalisation of the telecommunication industry coupled with the widespread deployment of intelligent network services is placing new demands on the signalling network at an ever increasing rate. Often there is a shortfall between what the existing network elements of the signalling system can provide in the short term and the demand for new services; this is in part due to the substantial expense and time involved to modify and requalify the operating software of major elements such as SSPs, SCPs and STPs.
As an example of the sort of problem encountered, a new customer service may result in heavier than expected loading of the relatively few SCPs in a network with the result that the SCPs present a potential bottleneck. Whilst more SCPs could be provided to handle the extra service requests, this is an expensive solution and one requiring long term planning. What is required is a way of at least temporarily increasing the service request handling capacity of the signalling system without massive investment and planning. Another example is the massive investment currently being required in the USA to support local number portability; implementation of this service according to the most commonly accepted solution calls for major changes to the SSP software to effect database lookups for ported numbers. Again what is required is a solution not involving massive upgrading of existing network elements.
In an attempt to deal with the SCP bottleneck problem referred to above, it has been proposed in EP-A-0 669 771 to provide a message interceptor for intercepting messages sent to an SCP to selectively suppress the messages or modify them (for example, to effect syntax translation or decryption) before forwarding the messages to the SCP. The message interceptor thus serves to relieve the SCP of some of its processing tasks and thereby avoid congestion.
FIG. 6 shows one embodiment of the message interceptor described in EP-A-0 669 771. The message interceptor is inserted in a link 52A, 53A, 52B, 53B with each link half 52A, 52B; 53A, 53B being terminated at a corresponding interface 50; 51 and MTP level-2 protocol engine 54; 55. The two level-2 protocol engines 54, 55 are connected through transfer circuits 56 that comprise MTP level 3 functionality 58 receiving both MSU data and link status information from the protocol engines. MSU data related to signalling network management and maintenance are identified (Service Indicator value less than 3) and handled entirely within the MTP level-3 block 58, these data being acted upon if addressed to the message interceptor itself as indicated by a match between the destination point code in the routing label and the signalling point code allotted to (and stored by) the message interceptor. MSU data related to higher levels are passed up to interception functionality (blocks 59). These blocks 59 contain the message interception functionality for selectively modifying or suppressing messages. Thus, each block 59 selectively acts on the data it receives and, where appropriate, then passes data (which may include response data) back to the MTP level-3 block 58 for transmission to the appropriate destination.
A key characteristic of this message interceptor is that it operates separate level-2 links with the two signalling points at either end of the original link in which the interceptor has been inserted. These two links run by the interceptor will have different states and therefore it is not possible simply to bypass the message interceptor in the event of failure. Instead, any failure must be handled as for the failure of any signalling point by the relevant MTP level 3 mechanisms in a way which will be evident to other signalling points, particularly those at either end of the original link. The message interceptor is therefore not transparent and will affect the surrounding network, at least on failure.
It is an object of the present invention to provide apparatus that can be used to modify signalling messages on a link but which is less intrusive than that of the prior art.